The influence of the season on the levels of activities in crops following a short-term deposition of radionuclides to agricultural land

Transcription

1 The influence of the season on the levels of activities in crops following a short-term deposition of radionuclides to agricultural land Gerhard Proehl Division of Transport Radiation and Waste Safety Seminar Expert Programme of Environmental Management and Prognosis of Nuclear Emergencies Tsukuba (Japan), 9 July 2017 International Atomic Energy Agency

2 Exposure to people from releases of radionuclides to the environment

3 Contamination routes for plant products A Short-term 1 Direct deposition onto edible parts of plants 2 Deposition onto leaves -> transport to the edible parts 2 1 B Long-term 3 Deposition on soil and uptake through the roots 4 Resuspension of dust and redeposition on leaves and fruits 3 4

4 Food processing and preparation Crops can be directly ingested Leafy vegetables: Fruit vegetables (tomato, cucumber, etc.) Fruit (peach, apples, pear plum) Products may be processed Cereals -> bread Milk -> butter, cheese

5 Transfer to animal products Use of contaminated fodder Transfer to meat, milk, eggs

6 Radioecological model Radionuclides release to the atmosphere Activity in air Activity in rain Plants Soil Animals External exposure Inhalation Ingestion External exposure Radiation exposure of man

7 Purpose of radioecological model: What do we want to know? Activity levels in environmental media and related exposures Air Exposure through inhalation of radionuclides Soil Long-term uptake of radionuclides by crops External exposure from radionuclides deposited on the ground Plants Internal exposure through the intake of plant food products Intake of radionuclides by domestic animals Animals Internal exposure through the intake of plant food products

8 Which factors are determining the exposure? Radiological characterization Radionuclides deposited, Deposition per unit area Dry deposition or deposition with rain Environmental characterization Ability of soils to sorb/fix caesium Agricultural practice (e.g. use of fertilizer) Season of the deposition

9 Which factors are determining the exposure (cont.)? Life style and economic situation Do people produce their own food? If yes, to which extent? Where does the food come from? From the region? From the own country? From global suppliers? Spectrum of foods Aquatic food Terresstrial food Information Publication of results of food monitoring Provide information on radiation risks

10 Model processes and parameters Key processes following direct deposition on the leaves (1 st year) Dry deposition of radionuclides to soil and vegetation Leaf area Interception of wet deposited radionuclides by vegetation Leaf area Amount of rainfall Growth dilution and weathering loss from vegetation Transport of radionuclides in plants to the edible parts Key processes following deposition on soil (following years) Uptake of radionuclides by plants from soil Migration and fixation of radionuclides in soil Intake of radionuclides by domestic animals Transfer of radionuclides to meat, milk and eggs Modification of activity levels in foods during processing and culinary preparation.

11 Seasonality of growth: Development of wheat

12 Development of leaf area and biomass MÜLLER, H. PRÖHL, G., ECOSYS 87: A dynamic model for assessing radiological consequences of nuclear accidents, Health Phys. 64 (1993)

13 Rainfall and deposition Rainfall during the passage of the plume Increase of deposition to the ground due to washout Increase of deposition with increasing rainfall (intensity) Fraction of activity retained by crops (interception) decreases with amount of rainfall increases with the development of crops highest during the peak season Interception factor 1,000 0,900 0,800 0,700 0,600 0,500 0,400 0,300 Interception for Cs Leaf area index 1 1,5 2 2, ,200 0,100 0,000 0, Rainfall (mm)

14 Comparison of foliar and root uptake of caesium Root uptake: Deposition: Bq/m 2 Ploughing depth: 0.25 m Soil density: 1400 kg/m 3 => Activity (soil) = Deposition = Bq/m 2 = 29 Bq/kg Ploughing depth soil density 0.25 m 1400 kg/m 3 Transfer factor soil plant: 0.01 Bq/kg plant per Bq/kg soil Activity (plant) = 29 Bq/kg * 0.01 Bq/kg plant p. Bq/kg soil = 0.29 Bq/kg

15 Comparison of foliar and root uptake of caesium II Foliar uptake by cereals (wheat, rice, etc.) Deposition: Bq/m 2 Deposition date (in this example): 4-8 weeks before harvest Interception: Fraction of activity deposited which is retained by the plant 1) Dry deposition: ~ 1 (equivalent to 100%) 2) Wet depostion: ~ 0.1 for a rainfall of 10 mm (equivalent to 100%) Translocation: Fraction of caesium which is transported from the leaves to the grain (@ 4-8 weeks before harvest): ~ 10% Yield: 0.5 kg/m 2 Activity (wheat) = Deposition Interception factor Translocation factor Yield Bq m = 2000 Bq/kg (dry deposition) 0.5kg/m 2 = Bq m kg/m2 = 200 Bq/kg (wet deposition@10 mm rainfall)

16 Comparison of foliar and root uptake of caesium II Quantity Uptake through leaves, Deposition: Bq/m 2 6 weeks before harvest 1 st year Uptake from soil Deposition: Bq/m 2 Following years Dry deposition Wet deposition, 10 mm rain Activity in cereals 2000 Bq/kg 200 Bq/kg 0.3 Bq/kg The values are rough estimates: Real values depend on the actual circumstances. The comparison illustrates the importance of the contamination process.

17 Comparison of activities in potatoes and wheat following different dates of deposition Potatoes Winter wheat Foliar uptake Foliar uptake Root uptake Root uptake Starting point of the simulation is a time-integrated Cs-137 activity in air of 1000 Bq h/m 2. Depending on the actual stage of development, this leads to a deposition on the foliage of ca Bq/m 2.

18 Seasonality of Cs-137 in milk following the Chernobyl fallout in spring (UNSCEAR 2008, dairy farm near Munich, Germany) Weathering loss Growth dilution Half-life: 14 days Transition from winter to summer feeding Feeding of hay prepared in spring/summer 30 April 1986

19 Result: time-dependent activity in foodstuffs 1,0E+3 ECOSYS: Activity in foodstuffs after deposition on 1st May Activity in foodstuffs (Bq/kg 1,0E+2 1,0E+1 1,0E+0 1,0E-1 leafy veget. milk pork potatoes 1,0E Apr 02. Jul 02. Okt 02. Jan 04. Apr 05. Jul 05. Okt Time

20 Cs-137 whole body counting (near Munich) ECOSYS Model Males BfS Females BfS Males GSF Females GSF Maximum Minimum Müller, H, and Pröhl, G., The radioecological Model ECOSYS: Concept and Applications, Proc. Intern. Workshop on Improvement of Environmental Transfer Models and Parameters, Tokyo, Japan, 5-6 February, 1996

21 Summary The uptake of radionuclides through leaves may be very effective Subject to pronounced seasonal variations in the plants development stage The uptake through leaves is much more effective than uptake through the roots. Crops grow during different time slots within the overall vegetation period Activity levels in different products may vary substantially, because the crops may be affected at very different stages of development. The radiological evaluation of radionuclide depositions occurring during the vegetation periods requires The careful characterization of the environmental conditions of the affected areas The growing periods of crops cultivated in the area The use of crops as food or feedstuffs The agricultural practices in animal husbandry

22 Conclusions What does this mean? 1000 Bq/kg Cs-137 Levels in food other than infants foods for international trade as provided by the Codex Alimentarius Bq/m 2 Cs-137-deposition during the peak season Cereals: Bq/m 2 (10 kbq/m 2 ) would cause Dry deposition: 2000 Bq/kg Wet deposition (10 mm rain): 200 Bq/kg

23 Total Cs-137 deposition in Europe

24 . Deposition density map for Cs-137 Kimiaki Saito, et al: Detailed deposition density maps constructed by large-scale soil sampling for gamma-ray emitting radioactive nuclides from the Fukushima Dai-ichi Nuclear Power Plant accident, Journal of Environmental Radioactivity, Volume 139, 2015,

25 Conclusions (cont.) The Chernobyl and the Fukushima accidents occurred early in the year, during the transition of winter to spring: Only few crops were affected by direct deposition of radionuclides. Implications for agriculture and trade remained relatively little. Large-scale depositions during the peak growing season could have much larger radiological implications Activity levels in crops would be much larger Areas potentially affected by restrictions would be much larger (if the same radiological criteria are applied) Implications may need to be explored to elaborate solid grounds for contingency planning.

26 Conclusions (cont.) The Chernobyl and the Fukushima accidents occurred early in the year, during the transition of winter to spring: Only few crops were affected by direct deposition of radionuclides. Radiocaesium was taken up by the crops in most cases through the roots only. Caesium levels in food and feed remained relatively low, even in the years 1986 and 2011 respectively Implications for agriculture and trade remained relatively little